1. Field of the Invention
The present invention relates to an electrophotographic photoconductor (hereafter, also simply referred to as “photoconductor”), to a manufacturing method thereof, and to an electrophotographic device comprising the electrophotographic photoconductor. More particularly, the present invention relates to an electrophotographic photoconductor that is used in electrophotographic devices such as copiers, fax machines and printers, to a manufacturing method of the electrophotographic photoconductor, and to an electrophotographic device comprising the electrophotographic photoconductor.
2. Background of the Related Art
Image forming methods that rely on electrophotography are widely used in copiers, printers, plotters and digital multifunction machines that combine the functions of the foregoing, not only for office use, but also, in recent years, for personal use in the form of, for instance, small printers and fax machines. Many types of photoconductors for these electrophotographic devices have been developed since the invention by Carlson (Patent Document 1). Organic photoconductors (OPC) that employ organic materials are ordinarily used nowadays as photoconductors.
Known photoconductors include separate-function photoconductors, which are made up of a sequential stack of an undercoat layer that comprises a conductive substrate of aluminum or the like having an anodized film or a resin film thereon; a charge generation layer resulting from dispersing a photoconductive organic pigment, such as phthalocyanines or azo pigments, in a resin; a charge transport layer resulting from dispersing, in a resin, molecules having a partial structure that contributes to charge hopping conduction, such as amine or hydrazone molecules bonded to π electron conjugated systems; and a protective layer, as the case may require. Other known photoconductors include, for instance, single layer-type photoconductors, wherein a single photosensitive layer, combining the functions of charge generation and charge transport, is provided on an undercoat layer, as the case may require.
For the sake of mass productivity, the above layers are ordinarily formed in accordance with a method wherein a pigment (charge generation material) having a charge generation or light scattering function, or a charge transport material having a charge transport function, are respectively dissolved or dispersed in an appropriate resin solution to yield a coating material in which a conductive substrate is then dipped.
In recent years, so-called digital devices have become mainstream electrophotographic devices. These devices, which use a monochromatic exposure light source in the form of, for instance, argon, helium-neon, a semiconductor laser or a light-emitting diode, digitalize information such as images and text, to convert the information into optical signals, on the basis of which light is irradiated into a charged photoconductor to form thereon an electrostatic latent image that is then made visible by way of toner.
Among charge generation materials, phthalocyanines exhibit greater light absorption at the oscillation wavelength range of semiconductor lasers (780 nm), as compared with other charge generation materials, and exhibit likewise excellent charge generation ability. As a result, phthalocyanines are widely used as a material for photosensitive layers. Known current photoconductors employ various phthalocyanines having a central metal of copper, aluminum, indium, vanadium, or titanium.
Photoconductor charging methods include, for instance, contactless charging methods in which a charging member such as a scorotron does not come into contact with the photoconductor, and contact charging methods in which a charging member that uses a semiconductive rubber roller or brush comes into contact with the photoconductor. Contact charging methods are advantageous, as compared with contactless charging methods, in that corona discharge takes place very close to the photoconductor. As a result, little ozone is generated, and the applied voltage may be lower. Therefore, contact charging methods allow realizing electrophotographic devices that are more compact, less expensive and environmentally less polluting, for which reason they have become the mainstream charging method in medium to small devices.
Means for cleaning the photoconductor surface involve mainly, for instance, plate scraping or cleaning simultaneous with developing. In plate cleaning, untransfered residual toner of the surface of the surface of the organic photoconductor is scraped by a plate, and the toner is recovered into a waste toner box or is returned to the developing device. Cleaners that employ plate scraping need space for a recovery box for recovered toner, and/or space for recycling. Also, the filling state of the toner recovery box must be monitored. The life of the electrophotographic photoconductor may be shortened through damage of the surface of the organic photoconductor when paper dust or external additive remains on the plate. Therefore, the developing process may be accompanied by toner recovery, or by a process of adsorbing, magnetically or electrically, residual toner adhered to the surface of the electrophotographic photoconductor, immediately in front of the developing roller. In order to increase toner transfer efficiency in the transfer step, control is performed so as to optimize the transfer current in accordance with the temperature and humidity of the environment and in accordance with paper characteristics. Better transfer efficiency results in less residual toner. Organic photoconductors suitable for the above-described processes and contact charging must have improved toner releasability (Patent Document 2) and/or exhibit little transfer influence.
In reverse development processes, dark potential corresponds to white paper portions on the image, while bright potential corresponds to text portions. Therefore, the presence of structural defects on the substrate, such as significant unevenness, or defects derived from substance inhomogeneity, on account of impurities or the like, is visualized in the form of image defects such as ground fogging or black spots on white paper portions. Defective images are thought to occur on account of injection of charge from the conductive substrate into the photosensitive layer, caused by defects in the conductive substrate, since these charge injections give rise to local drops in charging potential at defect sites. In electrophotographic devices that employ simultaneously a reverse developing method and a contact charging method, in particular, direct contact between the photoconductor and the charging member may result in charge leakage. Such electrophotographic devices are highly prone to suffering the above problem. Also, color machines have been gaining in popularity in recent years. Color machines, however, have often a high transfer current setting, and are therefore likelier to exhibit undesirable charge leakage during transfer.
An ordinary way of improving on the problems of such electrophotographic devices is to provide an undercoat layer between the conductive substrate and the photosensitive layer. The undercoat layer uses an aluminum anodized film, a boehmite film, or a resin film of, for instance, polyvinyl alcohol, casein, polyvinyl pyrrolidone, polyacrylic acid, gelatin, polyurethane, polyamide or the like. For instance, Patent Document 3 discloses a photoconductor using an anodized film in an undercoat layer. Patent Documents 4 to 6 disclose photoconductors having undercoat layers that comprise specific nylon types. These undercoat layers, however, suffer from a problem to be solved, namely image defects caused by interference fringes derived from reflection of exposure light by the substrate.
Copolymer nylon films are obtained as uniform films by dip coating, are excellent for mass production and are inexpensive, for which reasons they are widely employed. For instance, Patent Document 7 discloses the feature of using caprolactam as a constituent monomer of a copolymer nylon resin in a photoconductor for rear-face exposure. Also, Patent Document 8 indicates that an undercoat layer comprising a nylon resin having a specific copolymer composition affords excellent charging and residual potential characteristics. Patent Document 9 indicates that a photoconductor coating solution comprising a copolymer polyamide resin having a specific diamine component is effective for enhancing coatability and storage stability. The electric characteristics of the above undercoat layers vary significantly depending on the use environment, and give rise to problems of ground fogging in the image due to fluctuations in electric resistance caused by moisture absorption by the undercoat layer, in particular in high-temperature high-humidity environments. A further problem in low-temperature low-humidity environments is the occurrence of exposure memory on the image caused by charge traps in the film on account of lower density or higher resistance in the undercoat layer, as a result of an increased bright potential, owing to a significant increase in resin resistivity.
To counteract the occurrence the above image problems, it has been proposed to prevent the appearance of image defects, caused by interference fringes, by suppressing excessive reflection of exposure light by the substrate, and to use, as the undercoat layer, a layer in which metal oxide particles, such as titanium oxide, zinc oxide or the like, are dispersed in a resin, with a view to appropriately adjusting the resistance value of the undercoat layer. For instance, Patent Document 10 discloses the feature of using a resin layer containing titanium oxide in an interlayer, with a view to curbing environmental dependence. Patent Document 11 indicates that moisture resistance can be enhanced by using a polyamide resin having a specific structure, in an interlayer. Patent Document 12 discloses a photoconductor comprising an azo pigment and a copolymer polyamide resin having a diamine component of specific structure. Patent Document 13 discloses a photoconductor that uses a polyamide resin obtained by condensation of polymer fatty acids and diamines.
Other factors that lead to image defects such as ground fogging and black spots on a white background include, for instance, formation of aggregates of the metal oxide that is used in the undercoat layer. When present in the coating solution, such aggregates become charge paths in the film upon application of the coating solution. These charge paths give rise in turn to micro-leaks of charge on the photosensitive layer surface, that result in image defects similar to those caused by ground defects. Patent Document 14 indicates that scumming caused by long-term use can be curtailed by using a photoconductor that employs a metal oxide, a specific copolymer and a phthalocyanine pigment.
Patent Document 15 discloses a photoconductor in which an undercoat layer uses a polyamide resin containing an aromatic dicarboxylic acid monomer, as a photoconductor having good him characteristics and good metal oxide dispersibility, in order to improve environment dependence.
Patent Document 1: U.S. Pat. No. 2,297,691
Patent Document 2: Japanese Patent Application Laid-open No. 2006-39022
Patent Document 3: Japanese Patent Application Laid-open No. 2002-323781
Patent Document 4: Japanese Patent Application Laid-open No. H5-34964
Patent Document 5: Japanese Patent Application Laid-open No. H2-193152
Patent Document 6: Japanese Patent Application Laid-open No. H3-288157
Patent Document 7: Japanese Patent Application Laid-open No. S60-501723
Patent Document 8: Japanese Patent Application Laid-open No. H8-328283
Patent Document 9: Japanese Patent Application Laid-open No. H4-31870
Patent Document 10: Japanese Patent Application Laid-open No. S63-298251
Patent Document 11: Japanese Patent Application Laid-open No. 2003-287914
Patent Document 12: Japanese Patent Application Laid-open No. 2006-208474
Patent Document 13: Japanese Patent Application Laid-open No. 2006-221157
Patent Document 14: Japanese Patent Application Laid-open No. 2007-178660
Patent Document 15: Japanese Patent Application Laid-open No. 2004-101699
However, Patent Document 10 discloses only an example of a nylon resin having a specific structure. Patent Document 11 does not mention any aromatic ring in the dicarboxylic structure of the constituent monomers, and does delve sufficiently into the effect that is elicited by adding an aromatic dicarboxylic acid as a monomer.
Patent Document 12 discloses a photoconductor comprising an azo pigment and a copolymer polyamide resin having a diamine component of specific structure, but does not disclose the effect of the structure on the transfer history of the polyamide resin. Patent Document 13, which discloses a photoconductor that uses a polyamide resin obtained by condensation of a polymer fatty acid and a diamine, is problematic as regards fluctuation in the properties of the undercoat layer caused by oxidation of unsaturated fatty acids in the coating solution.
Patent Document 14 indicates that scumming caused by long-term use can be curtailed by using a photoconductor that employs a metal oxide, a specific copolymer and a phthalocyanine pigment. Although the polymer resin disclosed in Patent Document 14 can inhibit the generation of secondary aggregates, there is missing a thorough appraisal on potential fluctuations caused by transfer influence in devices having high transfer currents, such as color machines.
Patent Document 15, which proposed a photoconductor in which a polyamide resin containing aromatic dicarboxylic acid monomers is used in an undercoat layer, was problematic as regards the occurrence, when such an undercoat layer is employed, of uneven density in the image, on account of the influence on transferability of a high-transfer current process, as is the case in four-cycle color machines.
Conventional photoconductors, therefore, failed to avoid the problem of image defects in the form of memory or density changes in transfer sites in a subsequent process, and which arose from accumulation of reverse-polarity space charge in the photosensitive layer, and from the resulting negative influence on charging characteristics upon a subsequent rotation process, in cases of high transfer current settings, as found in color machines.
In the light of the above, thus, it is an object of the present invention to provide an electrophotographic photoconductor having good coating solution stability and good metal oxide dispersibility that is free of image defects such as ground fogging and black spots on a white background, and that affords good image characteristics in various environments.
A further object of the present invention is to provide an electrophotographic photoconductor having good image gradation properties and color reproducibility, in particular in color machines.
A further object of the present invention is to provide an electrophotographic photoconductor that affords good image quality stably, i.e. an electrophotographic photoconductor that affords high image homogeneity and in is free of transfer history in the form of image memory, by precluding potential fluctuations on account of transfer influence, also in devices having high transfer current, such as color machines.
Yet another object of the present invention is to provide a method for manufacturing the above electrophotographic photoconductor and to provide an electrophotographic device comprising the electrophotographic photoconductor.